Propulsion modules have been adapted to cope in earth's atmosphere. However what are some of the differences that would have to be taken into account if one was ever to fly in a thicker or thinner atmosphere. Would you need the same amount of energy? bigger engines? longer propellers (in case of a helicopter). etc..
Conventional Earthly jet aircraft already cope with a wide range of atmospheres -- from sea level pressure ("1 atmosphere") to 20% atmosphere at 35,000 feet (a typical airliner cruise altitude) to 10% atmosphere for high-performance military jets at 60,000 feet. As the atmosphere gets thinner, your jet engines have less oxygen to burn, and your wings provide less lift, but the plane is contending with less atmospheric drag. To get much higher than that, you need very large wings or very high speeds to maintain lift in a winged plane (e.g. the U-2 or the SR-71), or you switch to high powered rocket engines and forego wings entirely, becoming a spacecraft.
For thicker atmospheres, staying in the air is easy, but going fast is hard. If air pressure is higher, you can get the same amount of lift at lower speed or with smaller wings, but drag increases with the density of the air, so you'd be more efficient going very slowly. But, of course, if the air is that thick at the ground, you can always just go up! A plane designed for Earthly atmospheres would work just fine in a moderately denser atmosphere (assuming a reasonable amount of oxygen, and no unexpected pollutants) -- it would just have a higher flight ceiling.